Power grids are electric networks including power generation equipment, substation equipment, switchgear equipment, and loads, to generate power and supply the power from the power generation equipment to the loads. The power generation equipment of a power grid comes in various forms in accordance with particular kinds of energy sources used to generate electricity. Among the various types of power generation equipment, intermittent renewable energy generation equipment, including wind power, photovoltaics, geothermal energy, biomass energy, or other renewable energy, is estimated to significantly increase in percentage terms in the near future. Since the intermittent renewable energy generation equipment contains elements prone to cause instability of the respective power grids, these facilities need to incorporate a variety of measures for assuring quality of the electric power supplied from the power grids.
JP 4053965 B, for example, discloses a method of controlling a local grid obtained by interconnecting, as power supply equipment, a wind power generator and a photovoltaic generator (intermittent renewable energy), a gas/diesel power generator (a power device with low environmental acceptability), and a gas turbine power generator (a power device with high environmental acceptability), and further interconnecting, as a thermoelectric load, a wide-area small-power load (e.g., a unit of houses) and a centralized power load (e.g., an office building). The control method described in JP No. 4053965 is a technique intended to realize stable operation of a power grid under irregular power supply from the intermittent renewable energy. In this conventional control method, if a power demand exceeds the amount of power generated by the wind power generator and photovoltaic generator, the gas/diesel power generator and the gas turbine power generator are started up to initially supply power from the gas/diesel power generator and subsequently start using power from the gas turbine generator. In the event that the power demand further increases afterwards, a switchgear is closed to make power be supplied from a busbar. The gas turbine generator is operated in this way to meet a particular thermal load demand and then supply necessary amount of heat from an heat recovery system to a heat transport facility.
If intermittent renewable energy generation equipment is present in the power grid, a system disturbance such as a momentary power failure could result in simultaneous parallel off of all pieces of the intermittent renewable energy generation equipment from the power grid. This kind of event can be a big obstacle to stable operation of power grids when intermittent renewable energy generation equipment proliferates in greater quantities. For this reason, intermittent renewable energy generation equipment will be required to have, as a system interconnection requirement, a fault-ride-through (FRT) capability that causes no simultaneous parallel off in case of a system disturbance such as a momentary power failure. The equipment satisfying such a requirement is expected to be of high demand in the future.
A technique associated with the above requirement is proposed in JP 2005-51867 A, the patent of which is intended to provide a wind power generating system capable of preventing acceleration step out (loss of synchronism) and power instability, both due to system troubles, and achieving continued stable supply of electric power. The particular wind power generating system includes an induction generator, detection devices, an alternating-current energization generator, and a control device. The induction generator generates electricity from a rotating force based on wind power. The detection devices detect a rotating speed and voltage of the generator. The alternating-current energization generator connected in parallel to the induction generator is adapted to control reactive power when necessary. The control device performs secondary excitation control of the alternating-current energization generator at a time of the detection of an abnormal rotating speed or voltage by at least one of the detection devices. In case of abnormality, the generating system feeds reactive power into the induction generator to control the generator to operate at stabilized rotating speed and voltage.